This invention relates to methods for preparing new crystalline titanium molecular sieve zeolite compositions. More particularly, the invention is directed to improved variants of ETS-4, and methods of forming same.
Since the discovery by Milton and coworkers (U.S. Pat. Nos. 2,882,243 and 2,882,244) in the late 1950""s that aluminosilicate systems could be induced to form uniformly porous, internally charged crystals, analogous to molecular sieve zeolites found in nature, the properties of synthetic aluminosilicate zeolite molecular sieves have formed the basis of numerous commercially important catalytic, adsorptive and ion-exchange applications. This high degee of utility is the result of a unique combination of high surface area and uniform porosity dictated by the xe2x80x9cframeworkxe2x80x9d structure of the zeolite crystals coupled with the electrostatically charged sites induced by tetrahedrally coordinated Al+3. Thus, a large number of xe2x80x9cactivexe2x80x9d charged sites are readily accessible to molecules of the proper size and geometry for adsorptive or catalytic interactions. Further, since charge compensating cations are electrostatically and not covalently bound to the aluminosilicate framework, they are generally base exchangeable for other cations with different inherent properties. This offers wide latitude for modification of active sites whereby specific adsorbents and catalysts can be tailormade for a given utility.
In the publication xe2x80x9cZeolite Molecular Sievesxe2x80x9d, Chapter 2, 1974, D. W. Breck hypothesized that perhaps 1,000 aluminosilicate zeolite framework structures are theoretically possible, but to date only approximately 150 have been identified. While compositional nuances have been described in publications such as U.S. Pat. Nos. 4,524,055; 4,603,040; and 4,606,899, totally new aluminosilicate framework structures are being discovered at a negligible rate.
With slow progress in the discovery of new aluminosilicate based molecular sieves, researchers have taken various approaches to replace aluminum or silicon in zeolite synthesis in the hope of generating either new zeolite-like framework structures or inducing the formation of qualitatively different active sites than are available in analogous aluminosilicate based materials.
It has been believed for a generation that phosphorus could be incorporated, to varying degrees, in a zeolite type aluminosilicate framework. In the more recent past (JACS 104, pp. 1146 (1982); proceedings of the 7th International Zeolite Conference, pp. 103-112, 1986) E. M. Flanigan and coworkers have demonstrated the preparation of pure aluminophosphate based molecular sieves of a wide variety of structures. However, the site inducing Al+3 is essentially neutralized by the P+5, imparting a +1 charge to the framework. Thus, while a new class of xe2x80x9cmolecular sievesxe2x80x9d was created, they are not zeolites in the fundamental sense since they lack xe2x80x9cactivexe2x80x9d charged sites.
Realizing this inherent utility limiting deficiency, for the past few years the research community has emphasized the synthesis of mixed aluminosilicate-metal oxide and mixed aluminophosphate-metal oxide framework systems. While this approach to overcoming the slow progress in aluminosilicate zeolite synthesis has generated approximately 200 new compositions, all of them suffer either from the site removing effect of incorporated P+5 or the site diluting effect of incorporating effectively neutral tetrahedral +4 metal into an aluminosilicate framework. As a result, extensive research in the research community has failed to demonstrate significant utility for any of these materials.
A series of zeolite-like xe2x80x9cframeworkxe2x80x9d silicates have been synthesized, some of which have larger uniform pores than are observed for aluminosilicate zeolites. (W. M. Meier, Proceedings of the 7th International Zeolite Conference, pp. 13-22 (1986)). While this particular synthesis approach produces materials which, by definition, totally lack active, charged sites, back implantation after synthesis would not appear out of the question although little work appears in the open literature on this topic.
Another and most straightforward means of potentially generating new structures or qualitatively different sites than those induced by aluminum would be the direct substitution of some charge inducing species for aluminum in a zeolite-like structure. To date the most notably successful example of this approach appears to be boron in the case of ZSM-5 analogs, although iron has also been claimed in similar materials. (EPA 68,796 (1983), Taramasso, et. al.; Proceedings of the 5th International Zeolite Conference; pp. 40-48 (1980)); J. W. Ball, et. al.; Proceedings of the 7th International Zeolite Conference; pp. 137-144 (1986); U.S. Pat. No. 4,280,305 to Kouenhowen, et. al. Unfortunately, the low levels of incorporation of the species substituting for aluminum usually leaves doubt if the species are occluded or framework incorporated.
In 1967, Young in U.S. Pat. No. 3,329,481 reported that the synthesis of charge bearing (exchangeable) titaniumsilicates under conditions similar to aluminosilicate zeolite formation was possible if the titanium was present as a xe2x80x9ccritical reagentxe2x80x9d +III peroxo species. While these materials were called xe2x80x9ctitanium zeolitesxe2x80x9d no evidence was presented beyond some questionable X-ray diffraction (XRD) patterns and his claim has generally been dismissed by the zeolite research community. (D. W. Breck, Zeolite Molecular Sieves, p. 322 (1974); R. M. Barrer, Hydrothermal Chemistry of Zeolites, p. 293 (1982); G. Perego, et. al., Proceedings of 7th International Zeolite conference, p. 129 (1986)). For all but one end member of this series of materials (denoted TS materials), the presented XRD patterns indicate phases too dense to be molecular sieves. In the case of the one questionable end member (denoted TS-26), the XRD pattern might possibly be interpreted as a small pored zeolite, although without additional supporting evidence, it appears extremely questionable.
A naturally occurring alkaline titanosilicate identified as xe2x80x9cZoritexe2x80x9d was discovered in trace quantities on the Siberian Tundra in 1972 (A. N. Mer""kov, et. al.; Zapiski vses Mineralog. Obshch., pp. 54-62 (1973)). The published XRD pattern was challenged and a proposed structure reported in a later article entitled xe2x80x9cThe OD Structure of Zoritexe2x80x9d, Sandomirskii, et. al., Sov. Phys. Crystallogr. 24(6), Nov.-Dec. 1979, pp. 686-693.
No further reports on xe2x80x9ctitanium zeolitesxe2x80x9d appeared in the open literature until 1983 when trace levels of tetrahedral Ti(IV) were reported in a ZSM-5 analog. (M. Taramasso, et. al.; U.S. Pat. No. 4,410,501 (1983); G. Perego, et. al.; Proceedings of the 7th International Zeolite Conference; p. 129 (1986)). A similar claim appeared from researchers in mid-1985 (EPA 132,550 (1985)). The research community reported mixed aluminosilicate-titanium (IV) (EPA 179,876 (1985); EPA 181,884 (1985) structures which, along with TAPO (EPA 121,232 (1985) systems, appear to have no possibility of active titanium sites. As such, their utility, has been limited to catalyzing oxidation.
In U.S. Pat. No. 4,938,939, issued Jul. 3, 1990, Kuznicki disclosed a new family of synthetic, stable crystalline titaniumsilicate molecular sieve zeolites which have a pore size of approximately 3-4 Angstrom units and a titania/silica mole ratio in the range of from 1.0 to 10. The entire content of U.S. Pat. No. 4,938,939 is herein incorporated by reference. These titanium silicates have a definite X-ray diffraction pattern unlike other molecular sieve zeolites and can be identified in terms of mole ratios of oxides as follows:
1.0xc2x10.25 M2/nO:TiO2:YSiO2:ZH2O
wherein M is at least one cation having a valence of n, Y is from 1.0 to 10.0, and Z is from 0 to 100. In a preferred embodiment, M is a mixture of alkali metal cations, particularly sodium and potassium, and y is at least 2.5 and ranges up to about 5.
The original cations M can be replaced at least in part with other cations by well-known exchange techniques. Preferred replacing cations include hydrogen, ammonium, rare earth, and mixtures thereof. Members of the family of molecular sieve zeolites designated ETS-4 in the rare earth-exchanged form have a high degree of thermal stability of at least 450xc2x0 C. or higher depending on cationic form, thus rendering them effective for use in high temperature catalytic processes. ETS zeolites are highly adsorptive toward molecules up to approximately 3-5 Angstroms in critical diameter, e.g. water, ammonia, hydrogen sulfide, SO2, and n-hexane and are essentially non-adsorptive toward molecules which are larger than 5 Angstroms in critical diameter.
Members of the ETS-4 molecular sieve zeolites have an ordered crystalline structure and an X-ray powder diffraction pattern having the following significant lines:
The above values were collected using standard techniques on a Phillips APD3720 diffractometer equipped with a theta compensator.
A large pore crystalline titanium molecular sieve composition having a pore size of about 8 Angstrom units has also been developed by the present assignee and is disclosed in U.S. Pat. No. 4,853,202, which patent is herein incorporated by reference. This crystalline titanium silicate molecular sieve has been designated ETS-10.
The new family of microporous titanosilicates developed by the present assignee, and generically denoted as ETS, are constructed from fundamentally different building units than classical aluminosilicate zeolites. Instead of interlocked tetrahedral metal oxide units as in classical zeolites, the ETS materials are composed of interlocked octahedral chains and classical tetrahedral rings. In general, the chains consist of six oxygen-coordinated titanium octahedra wherein the chains are connected three dimensionally via tetrahedral silicon oxide units or bridging titanosilicate units. The inherently different crystalline titanium silicate structures of these ETS materials have been shown to produce unusual and unexpected results when compared with the performance of aluminosilicate zeolite molecular sieves. For example, the counter-balancing cations of the crystalline titanium silicates are associated with the charged titania chains and not the uncharged rings which form the bulk of the structure. In ETS-10, this association of cations with the charged titania chains is widely recognized as resulting in the unusual thermodynamic interactions with a wide variety of sorbates which have been found. This includes relative weak binding of polar species such as water and carbon dioxide and relatively stronger binding of larger species, such as propane and other hydrocarbons. These thermodynamic interactions form the heart of low temperature dessication processes as well as evolving Claus gas purification schemes. The unusual sorbate interactions are derived from the titanosilicate structure, which places the counter-balancing cations away from direct contact with the sorbates in the main ETS-10 channels.
In recent years, scores of reports on the structure, adsorption and, more recently, catalytic properties of wide pore, thermally stable ETS-10 have been made on a worldwide basis. This worldwide interest has been generated by the fact that ETS-10 represents a large pore thermally stable molecular sieve constructed from what had previously been thought to be unusable atomic building blocks.
Although ETS-4 was the first molecular sieve discovered which contained the octahedrally coordinated framework atoms and as such was considered an extremely interesting curiosity of science, ETS-4 has been virtually ignored by the world research community because of its small pores and reported low thermal stability. Recently, however, researchers of the present assignee have discovered a new phenomenon with respect to ETS-4. In appropriate cation forms, the pores of ETS-4 can be made to systematically shrink from slightly larger than 4 xc3x85 to less than 3 xc3x85 during calcinations, while maintaining substantial sample crystallinity. These pores may be xe2x80x9cfrozenxe2x80x9d at any intermediate size by ceasing thermal treatment at the appropriate point and returning to ambient temperature. These controlled pore size materials are referred to as CTS-1 (contracted titanosilicate-1) and are described in commonly assigned, U.S. Pat. No. 6,068,682, issued May 30, 2000. Thus, ETS-4 may be systematically contracted under appropriate conditions to CTS-1 with a highly controllable pore size in the range of 3-4 xc3x85. With this extreme control, molecules in this range may be separated by size, even if they are nearly identical. The systematic contraction of ETS-4 to CTS-1 to a highly controllable pore size has been named the Molecular Gate(trademark) effect. This effect is leading to the development of separation of molecules differing in size by as little as 0.1 Angstrom, such as N2/O2 (3.6 and 3.5 Angstroms, respectively), CH4/N2 (3.8 and 3.6 Angstroms), or CO/H2 (3.6 and 2.9 Angstroms). High pressure N2/CH4 separation systems are now being developed. This profound change in adsorptive behavior is accompanied by systematic structural changes as evidenced by X-ray diffraction patterns and infrared spectroscopy.
As synthesized, ETS-4 has an approximately 4 xc3x85 effective pore diameter. Reference to pore size or xe2x80x9ceffective pore diameterxe2x80x9d defines the effective diameter of the largest gas molecules significantly adsorbed by the crystal. This may be significantly different from, but systematically related to, the crystallographic framework pore diameter. For ETS-4, the effective pore is defined by eight-membered rings formed from TiO62xe2x88x92 octahedra and SiO4 tetrahedra. This pore is analogous to the functional pore defined by the eight-membered tetrahedral metal oxide rings in traditional small-pored zeolite molecular sieves. Unlike the tetrahedrally based molecular sieves, however, the effective pore size of the eight-membered ring in ETS-4 can be systematically and permanently contracted with structural dehydration to CTS-1 materials as above described.
The pores of ETS-4 formed by the eight-membered polyhedral TiO6 and SiO4 units are non-faulted in a singular direction, the b-direction, of the ETS crystal and, thus, fully penetrate the crystal, rendering the ETS-4 and the related contracted version, CTS-1, extremely useful for molecular separations whether in the liquid, or gaseous state. The crystal lattice structure of ETS-4, however, is highly faulted along the other two of the three-dimensional axes. These faulted a- and c-directions are permeated by various small channel systems which are not interpenetrating and which contain serious diffusion blocks. Due to the open channels along the b-direction, and faulting in the a- and c-directions of the ETS-4 framework, it has been proposed that ETS-4 can be described as an intergrowth of four polymorphs. Two of the polymorphs contain non-blocked 12-ring pores disposed along the c-direction of the unit cell of ETS-4. These twelve-ring pores of approximately 6 Angstroms in these two polymorphs are aligned into channels along the c-direction of the ETS-4 framework structure without faulting. An ETS-4 molecular sieve formed of, or enriched by either or both of these c-channel polymorphs would yield intermediate and large pore variants of ETS-4 and, importantly, provide a superior CTS-type separation agent, capable of separating molecules having a size range of from 6 down to 4 Angstroms or less. Such polymorph-enriched ETS-4 would have the ability to participate in a greatly increased number of adsorptive separations relative to that believed possible with ETS-4 and contracted versions thereof formed from a random ETS intergrowth.
In accordance with the present invention, an ETS-4 titanium silicate is formed comprising enriched amounts of polymorphs which contain channels formed from 12-ring pores, which channels inter-penetrate the framework of the ETS-4 along the c-direction. The c-channeled polymorph-enriched ETS-4 also contains the interconnecting 8-ring pores along the b-direction of the ETS framework. The interconnecting pores along both the b- and c-directions can be systematically contracted to CTS-type materials by thermal dehydration without destroying the crystal structure of the titanium silicate. The polymorph-enriched ETS-4 and its contracted versions can separate by adsorption a greater number of gaseous and liquid molecules from mixtures containing the same than previously thought using ETS-4. Thus, whereas ETS-4 formed from an intergrowth of randomly arranged polymorphs can provide separation of molecules ranging in size from 4 xc3x85 to about 2.5 xc3x85, the C-channeled polymorph-enriched ETS-4 is capable of separating molecules ranging in size from approximately 6 xc3x85 to 4 xc3x85.
The C-channeled polymorph-enriched ETS titanium silicate molecular sieve is formed in accordance with this invention by heating a reaction mixture containing a titanium source, a source of silica, a source of alkalinity, water and a wetting agent to a temperature of from about 100xc2x0 C. to 300xc2x0 C. for a period time ranging from 8 hours to 40 days while controlling the pH range within the range used for ETS-4 formation. It has been found that the addition of the wetting agent to the crystalline titanium silicate-forming mixture results in an ETS-4 molecular sieve which is enriched in either or both of the polymorphs which contain interconnecting pores of 3 xc3x85 to 4 xc3x85 in diameter along the b-direction of the framework and interconnecting pores of 4 xc3x85 to 6 xc3x85 along the c-direction of the ETS-4 framework.